First of all, it will be noted that, very often, modern vehicle integral bodies make it necessary for the slides to mount a slide vertically with respect to the floor on the seat outer side, while the seat inner side is supported by a horizontally placed slide. It thus results that the lower profile bears by its bottom on the vehicle floor and therefore supports, via various members, not only the weight of a passenger but also the efforts due to a sliding motion of the seat, which sliding motion is controlled by a declutching member of the slides, this declutching member being often placed at a top of the seat back. The effect of this arrangement is to create a considerable torsion torque and the inner horizontal slide is particularly biased and deformed, thereby creating jammings of the mobile portion with respect to the fixed portion. Thus, the seat is operated with difficulty, with a risk of being blocked in a position which is not chosen by the user and, should an impact occur, the slide, which should resist to considerable efforts, is very rapidly destroyed without absorbing a sufficient amount of energy, with an increased risk for the passenger sitting on the seat.